1. Field of the Invention
The present invention relates to PTC (positive temperature coefficient) compositions consisting of conductive polymer compounds to be used as a self-resetting overcurrent protection device utilizing the PTC characteristics thereof.
2. Description of the Prior Art
Carbon black, the specific surface area of which is increased at the time of its formation, is well known as is the method described in Japanese Patent Laid-Open No. 24455/1981. It is also known that highly conductive compositions can be obtained by combining such carbon black with resin.
Also known is conductive resin compositions as described in Japanese Patent Publication No. 35696/1973. Such composition include resin and carbon powder with large surface area which is a by-product produced by partly oxidizing a hydrocarbon in steam in order to obtain a gas mixture heavy in hydrogen and carbon monoxide.
It is also well known that conductive compositions can be obtained by blending carbon black having a large specific area as shown in table 1 with resin.
TABLE 1 ______________________________________ Carbon Particle Specific Black Diameter Surface Area (Grade) (Name of Manufacture) (m.mu.) (m.sup.2 /g) ______________________________________ Black Cabot Corporation 15 1475 Pearls 2000 Conductex Colombian Carbon 20 1066 40-220 Japan Ltd. Ketjen Ketjen Black Inter- 30 950 Black EC national Corporation ______________________________________
Highly conductive carbon black described in the references Japanese Patent Laid-Open No. 24455/1981 is characterized by a low volume resistivity .rho. at the time of being blended with resin and a small change of .rho. due to temperature. Therefore, this carbon black is unsuitable for use as a conductor for a self-resetting overcurrent protection device which takes advantage of its PTC behavior, i.e. the sharp change in .rho. in response to changes in temperature.
Conductive resin compositions described in Japanese Patent Publication No. 35696/1973 utilize carbon black with a large specific surface area. However, its purpose is to obtain conductive resin compositions with improved conductivity as well as high mechanical strength by means of blending only a small amount of carbon black.
Further, when carbon black with a large specific surface area, such as those mentioned in Table 1, is blended, in various content ratios, with resin (polyethylene) and shaped, any such shaped article has, as indicated in FIG. 17, high electrical conductivity with relatively low content of carbon black. As shown in Table 6, however, such composition presents a problem in that it has low PTC characteristics, i.e. the rate of drastic increase of resistivity occuring around the melting point of the resin in question.